Some molded products made of a plastic material employ two different types of resins, and for example, have a layered structure in which one of the resins or a resin of the surface layers of the molded product (the layer on the inner surface side and the layer on the outer surface side of the molded product) is layered, while the other of the resins or the resin for the middle layer of the molded product is also layered. Those products are molded in a manner such that the resin layered as the middle layer of the molded product serves to implement an intended function, and the layer is sandwiched between the layers of the resin for the surface layers of the molded product. Among multi-layered molded products, not only those having a layered structure with two types of resins formed in three layers but also those having a layered structure with three types of resins formed in five or more layers are also commercially available.
Such molded products having a layered structure with a plurality of layers may be formed by injection molding. To this end, in a known method as disclosed in Patent Literature 1, a hot runner nozzle is used to allow a plurality of types of molten resins to flow therethrough, and the hot runner nozzle serves to merge a molten resin for the surface layers of the molded product and a molten resin for the middle layer of the molded product and feed the resulting molten resins into a metal mold for molding.